Both rigid gas permeable, i.e. "RGP", and soft contact lenses have been available for many years. Each lens type offers certain advantages over the other. For example, RGP lenses are typically easier to clean, provide greater vision acuity, and permit for correction of astigmatism. Soft lenses on the other hand, typically offer greater comfort.
In an effort to achieve the advantages offered by both RGP and soft lenses, bimodulus lenses have been proposed. These lenses include a RGP core section with a soft edge section annularly attached about the core section. The core section comprises a conventional RGP material having a relatively high modulus and the edge section comprises a conventional soft lens material (e.g. a hydrogel), having a relatively low modulus. Such bimodulus lenses have enhanced comfort due to their soft edge section, while maintaining the aforementioned advantages provided by conventional RGP lenses. Examples of such bimodulus lenses are provided in International Applications WO/9406621 and WO 94/29756 and U.S. Pat. Nos. 3,619,044; 4,208,362; and 3,489,491.
A significant drawback to bimodulus lenses is their tendency to tear between the core and edge sections. Known techniques for attaching the core and edge sections of such lenses include thermoplastic lamination, grafting, and copolymerization between the polymeric materials of the core and edge sections. Unfortunately, none of these known techniques have produced lenses which adequately resist tearing between the edge and core sections.